The present invention relates to a coating method for forming at least one layer of coating film on the surface of a continuously conveyed long-size flexible band-like support by use of an extrusion-type coating apparatus.
Recently, in the fields of coating magnetic recording media or the like, studies and various proposals for applying a thin functional layer at a high speed have been made with the improvement of product performance.
For example, JP-B-4-5508, JP-B-5-8065 or the like discloses a coating method in which an extrusion-type coating head is pressed onto a flexible support.
FIG. 6 is a schematic view of a main part of an apparatus corresponding to a coating apparatus disclosed in JP-B-5-8065. A conventional extrusion-type coating head 10 disclosed therein is constituted by a front edge block 11 disposed close to an upstream web support roller (not shown) and having a convex-curved front edge surface 11a, a doctor edge block 12 disposed close to a downstream-side web support roller (not shown) and having a convex-curved doctor edge surface 12a receding as a whole with a slight difference in level from the front edge surface 11a, and a slot 14 for dividing the front edge block 11 and the doctor edge block 12 at a slight space so as to supply a coating composition A from a pocket portion 13 continuously.
Further, the coating head 10 has a size relation of xcex81 less than xcex82 less than 180xc2x0 between angles xcex81 and xcex82. The angle xcex81 is formed between a first tangent t1 touching the front edge surface 11a at a slot-outlet end portion of the front edge surface 11a and a second tangent t2 touching the doctor edge surface 12a at a rear end portion of the doctor edge surface 12a. The angle xcex82 is formed between the first tangent t1 and a third tangent t3 which is drawn from the slot-outlet end portion of the front edge surface 11a so as to touch the doctor edge surface 12a. In addition, the front edge surface 11a has such a relative position relationship that the front edge surface 11a is close to the lower surface of a flexible support W on the upstream-side web support roller side.
The flexible support W which has passed through the upstream-side web support roller is continuously conveyed in the direction of the arrow C toward the downstream-side web support roller so as to be wound on the front edge surface 11a, the outlet of the slot 14 and the doctor edge surface 12a sequentially. During this conveyance, a slight gap enough to prevent the coating composition A from overflowing from the front edge surface 11a to the upstream side is provided between the flexible support W and the front edge surface 11a. This gap is provided by the effect of a combination of various factors such as the supply rate and fluid pressure of the coating composition A supplied to the vicinity of the outlet of the slot 14, the web tension, the shapes of the front edge surface 11a and the doctor edge surface 12a based on the size relation of xcex81 less than xcex82 less than 180xc2x0, and so on.
In addition, a meniscus of the coating composition A is formed in the gap so as to prevent the invasion of foreign matters adhering to the flexible support w conveyed from the upstream side and the air entrained on the flexible support W. On the other hand, on the lower surface of the flexible support W leaving the rear end portion of the doctor edge surface 12a, a coating film the thickness of which is reduced to about 10 xcexcm in a wet state is obtained by the application effect of a trap formed, near a coating point, by the above-mentioned difference in level.
On the other hand, the coating head disclosed in JP-B-4-5508 is designed to have a front edge surface different from the above-mentioned front edge surface 11a in the extrusion-type coating head 10 disclosed in JP-B-5-8065. According to the coating head disclosed in JP-B-4-5508, a surplus of the coating composition A is made to overflow to the upstream outside of the coating head through a slight gap between the front edge surface and the flexible support W so that foreign matters adhering to the flexible support W and the air entrained on the flexible support W are prevented from invading a coating film of the coating composition A.
Further, in this coating head, two doctor edge blocks are made to be adjacent to a front edge block through a slot so that another coating composition is applied as another coating film onto the coating film of the coating composition A which is in a wet state on the lower surface of the flexible support W.
However, also in this method, it was difficult to cope with the case where it was intended to make a further coating layer thinner, for example, to be not thicker than 10 xcexcm in a so-called wet state.
This is because the free surface on the upstream side becomes unstable due to the reduction in thickness and the air is conspicuously mixed into the coating layer so that a failure such as so-called thin coating streaks or the like occurs.
Therefore, there is adopted a method in which the fluid pressure on the doctor edge is increased by reducing the curvature radius of the doctor edge or by increasing the tension of the flexible support so that the air is restrained from mixing into the coating film to some extent. On the contrary, however, there arose a problem that the free surface did not stay in a die-edge leading end portion but moved to the upstream side so that the film was made uneven.
In addition, with reduction of the thickness of the coating layer, the distance between the front edge and the flexible support, which is essentially kept to be extremely narrow is made smaller, so that it is inevitable that the flexible support is brought into contact with the front edge due to a minute change of speed or tension of the flexible support, the surface characteristic of the flexible support, and so on.
As a result, the support surface is shaved or is deformed locally so that there may occur a bad influence such as coating streaks, injures for the coating layer or the support itself, and so on.
On the other hand, in the coating head disclosed in JP-B-4-5508, as the speed is made higher and the coating layer is made thinner, the coating composition A unevenly overflows with unsteady vibration unless the flow rate of surplus of the coating composition A overflowing from the front edge surface to the upstream outside of the coating head and the distance between the flexible support W and the front edge surface are established properly. As a result, a streaky defect is apt to be caused by coating unevenness or invasion of entrained air or foreign matters into the coating film.
It is an object of the present invention to solve the foregoing problems in the conventional art and to provide a coating method in which coating films can be kept superior in quality, can be produced at a high speed, and can be reduced in thickness.
The foregoing object of the present invention is achieved by the following configuration.
(1) In a coating method in which an extrusion-type coating head having a front edge surface and at least one doctor edge surface is pressed onto a flexible support laid and running between support rollers so as to coat the flexible support with a coating composition ejected from at least one slot to thereby provide at least one layer of coating film; the coating film is provided in the state where 10% to 80% of first coating composition is made to overflow through a gap formed between the support and the front edge.
(2) In the coating method defined in the above aspect (1), the coating film is provided in the state where the shortest distance between a tangent touching an outer circumferential surface of an upstream support roller and a first doctor edge surface and the front edge surface in a front edge block is set to be not shorter than 10 xcexcm.
(3) In the coating method defined in the above aspect (1), the coating film is provided in the state where the first coating composition adjusted to have a static viscosity of not less than 1 poise and a viscosity of not more than 50 centipoises at a shear rate of 10,000 secxe2x88x921 is made to overflow.
In a coating method according to the present invention, a coating film is provided in the state where 10 to 80% of the discharge amount of a first coating composition is made to overflow through a gap formed between an above-mentioned support and an above-mentioned front edge surface. Accordingly, the front edge and the support are not in direct contact with each other so that not only the support is prevented from being shaved but also the air is not mixed into the coating film. As a result, thin and even coating can be performed. Then, the viscosity of the coating composition is important to keep the amount of the overflow of the coating composition uniform and stable. By increasing the so-called static viscosity measured by a Brookfield viscometer, an effect of preventing the air from mixing into the coating film at a doctor edge portion is obtained.
Further, an edge at the downstream end of the first doctor edge surface has an effect of preventing the disorder of the interface between a layer formed by the coating composition and another layer overlapping the layer.